This section introduces aspects that may be helpful in facilitating a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
With the increasing demand for data services, wireless systems tend to use more and more bandwidth and higher carrier frequencies. For example, Fifth Generation (5G) wireless access may be expected to embrace mm-Wave frequencies to provide multi-Gbps data rates, typically in femto, pico, or metro cell type of deployments. With deployment of a higher number of small cells costs for backhaul infrastructure are growing. Using wireless backhauls may reduce the costs but introduces further complexity and processing for the wireless backhaul links.
Some solutions use wired backhauls (e.g. coax or optical cables) for transferring the data of small cells to a central unit. In some scenarios a wired backhaul is not desired or not possible. In this case alternatives are wireless backhaul methods like mm-wave backhauls with multiple min-wave links or massive MIMO (Multiple-input-Multiple-Output) backhauls, where with the help of an array antenna multiple beams are transmitted to the individual small cells.
A wireless backhaul may introduce cross interference between the multiple radio links that use the same radio resources, which is reduced as much as possible using antenna arrays with a high number of antennas and corresponding complex processing. It is tried to reduce this cross interference as much as possible by increasing the directivity of the radio links, e.g. by increasing the antenna size of the mm-wave links or by increasing the number of antennas of the massive MIMO array antenna. It is desired to make the quality of the massive MIMO backhaul links good enough such that backhaul impairments do not significantly reduce the quality of the wireless access link.
As a result the effort for the massive MIMO wireless backhaul is high in order to reduce the cross interference of multiple backhaul links. A large number of antenna elements may be used, precise channel state information may be required and computational expensive beamforming algorithms (e.g. Zero Forcing (ZF)) are used. In some solutions, where analog RX-TX (Receive-Transmit) data of wireless cells are transported with a massive MIMO backhaul solution to a central unit that handles the data of multiple small cells, the cross interference of the backhaul link can reduce the quality of the wireless access links.
Document US 2014/0307702 A1 discloses a concept for a mobile communication system with a wireless backhaul using massive MIMO. Document US 2012/0300654 A1 describes a concept for channel estimation in a relaying scenario.